Which valve is safe to use?

QUESTION: I have a question on my IGO-540-A1A, an engine with a rated RPM of 3,400. When it comes time for an overhaul, S.I. 1037K calls for intake valve LW-12949.

This valve is prohibited from use in an HIO-360-D1A, an engine with a rated RPM of 3,200 by S.I. 1280C. The use of this valve is also cautioned in engines of rated RPMs of 2,900, stating that valve tip failure can occur with any overspeed condition.

My question is how can this valve be safely used in my engine of 3,400 RPM? The engine was originally built with 73117 intake valves, which are safe to use in all these engines.

McCarthy Air
McCarthy, Alaska

ANSWER: Gary, you’ve raised an interesting question. I really don’t have an answer for you. How’s that for honesty?

I will, however, offer some advice that may assist in deciding which valve is legal and may be used in your IGO-540-A1A. As we all know, and have heard many times from our friends with the FAA, it’s the “”approved manufacturer’s data”” that they require.

You mentioned in your inquiry Lycoming Service Instructions SI 1037K and SI 1280C, so I know you’ve done some background research. Even though the current revision of SI 1037 is now “”M”” you were right on track with where you looked for the proper information. It’s nice to see that there are folks out there who will crack the books that are published for that purpose.

While there is no mention in either of those publications as to why the LW-12949 intake valve is currently approved for use in the IGO-540-A1A, these publications do provide you with the “”approved manufacturer’s data”” the FAA requires.

My guess as to why the LW-12949 valve is approved for use in your engine is simply a matter of common sense. I cannot ever recall, in all of my years at Lycoming, hearing any reports of an IGO/IGSO-540 series of engines experiencing an overspeed condition.

That, however, can not be said of the HIO-360 series of engines that turn 2,900-3,200 RPM in helicopter applications, and that’s probably why this valve has never been approved for those applications. The fact that they are more likely to experience an overspeed is understandable because the crankshaft is connected indirectly to the rotor blades via a belt system unlike your engine, where the prop is connected directly to the reduction gear, which is connected directly to the engine crankshaft.

When it comes time to overhaul your IGO-540-A1A, I’d say you’ve got all the data you need to go with the large, rotating intake valve, part number LW-12949, in accordance with the latest revision of Lycoming Service Instruction SI 1037.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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